Moisture may collect in the cavities of structures, such as for example and without limitation houses, buildings and the like. This moisture may come from capillary transport, such as by wind-driven rain, by rain or other water leaking into the structure, by water vapor diffusion and fluid flows, such as airflow, through the wall(s) of the structure. As used herein, the term fluid refers generally to any substance tending to flow or conform to the outline of its container including any gas, such as for example air, or any liquid, such as for example water. Humidity is the amount of water vapor in the air, with water vapor being the gaseous form of water. Condensation occurs when water vapor changes from a gas to a liquid. Most of the humidity in outside air comes from evaporation of water from bodies of water, and from water vapor emitted by plants and animals. Humidity in air inside a structure is raised by such activities as cooking, bathing, doing laundry, growing plants and the like. The humidity of air inside a structure can be lowered by a dehumidifier and the use of exhaust fans in areas where water vapor is created, or raised by a humidifier. When the humidity inside a structure is greater than 50%, condensation of the water vapor can occur, leading to mold, rot, pest infestation, and the like. When air cools, it loses its ability to “hold” moisture. The dew point is a measure of how much water vapor is actually in the air, whereas the relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at a constant pressure and temperature. The dew point is the temperature to which air must be cooled to reach saturation, which is when condensation occurs, whereas the relative humidity is a percentage that indicates how saturated the air is. For example, a relative humidity of 50% means that the air contains half of the amount of moisture needed for saturation.
Generally, the second law of thermodynamics dictates that heat flows spontaneously from a hot body to a cool body. Therefore, a warm fluid, such as air, will move toward a cold body, until an equilibrium is reached. Thus, while relatively warm air outside the structure may move toward relatively cooler air or body inside the structure, referred to herein as infiltration, relatively warm air inside a structure may move toward relatively cooler air or body outside the structure, referred to herein as exfiltration. The relatively cooler walls or structures are exposed to temperature gradients by infiltration and exfiltration. The temperature gradients induce moisture flows, such as for example water vapor and liquid flows. The moisture content and the corresponding relative humidity in the porous materials inside a wall cavity are such that moisture starts redistributing inside the wall to the colder side due to the effects of the temperature gradient. Sinks that attract water vapor include surfaces having a temperature that chills the air coming in contact with the surface to the dew point, thereby causing condensation on the surface.
When relatively warm and humid air encounters a relatively colder surface, such as a window pane, water vapor diffusion may cause condensation on that surface, so long as the dew point temperature exists. Condensation generally may occur when the relative humidity inside the structure is above about 50%. The flow of fluid tends to be toward the coldest point in the structure, which is typically one or more of the windows. Thus, whether the fluid is infiltrating from outside to inside, as on a relatively hot day, or exfiltrating from inside to outside, as on a relatively cold day, condensation may occur on the window(s) and may drip down into the sill, causing damage to the structure.
For example, when conventional window frames and sashes are used in structures in which the temperature inside the structure is greater than the temperature outside the structure, heat transfer from portions of the frame and sash inside the structure may lower the temperature of those portions below the dew point of the air inside the structure, thereby causing moisture condensation on their inside surfaces. Conversely, if the temperature outside the structure is greater than the temperature inside the structure, then the heat transfer may lower the dew point of the air outside the structure, thereby causing moisture condensation on the outside surfaces. Such condensation may facilitate the formation of mold or otherwise cause damage to the structure.
To minimize this objectionable heat transfer, thermal barrier elements having a relatively low coefficient of thermal conductivity are commonly interspaced between inside portions of the window frame and outside portions of the frame. As used herein, the phrase coefficient of thermal conductivity or coefficient of heat conductivity means any coefficient indicating the rate of heat transmission through a given material. Such barriers are not only sometimes difficult to install properly, but do not always sufficiently minimize the heat transfer from the inside frame portions to prevent moisture condensation thereon. It is also possible to control the amount of moisture in the air inside the structure, such as by the use of a dehumidifier. However, a dehumidifier typically requires some type of electrical power to extract the water vapor from the air. What is needed is a generally passive, mechanical system for controlling moisture condensation without the need for any external power to cause or induce condensation.